1. Field of the Invention
The present invention relates to an optical information detection circuit and an optical pickup for detecting optical information recorded on an optical information recording medium by receiving the light returned from the optical information recording medium.
2. Description of the Related Art
There have recently been demands for an external storage device having a large capacity for storing a tremendous volume of information, particularly, image information or the like, with development of a computer system. Since a conventional magnetic disk used as an external storage device has a problem in that it is impossible to obtain a sufficient memory capacity and exchange mediums, various optical disk devices for storing information in optical disks have been developed.
Although such optical disk devices include a device for storing information in an additional storage type optical disk, this additional storage type optical disk is impossible to rewrite. A reloadable optical disk device such as a photomagnetic disk for photomagnetically storing information in a photomagnetic disk thus attracts attention as an external storage device.
In conventional reproduction means for reproducing information photomagnetically recorded by the photomagnetic disk device, the light reflected from a photomagnetic disk (not shown) is converged into a convergent flux by a converging lens through a beam splitter (not shown), and is caused to enter a polarized beam splitter 52, as shown in FIG. 8. The main axis of the polarized beam splitter 52 is inclined at 45.degree., and the polarized light entering is inclined at 45.degree.. The light 53 passing through the polarized beam splitter 52 is polarized light having a P polarized light component only, and reflected light 54 is polarized light having an S polarized light component only. These polarized light beams are received by photodetectors 55 and 56 and converted into electrical signals.
As illustrated in FIG. 9, current outputs 57 and 58 from the photodetectors 55 and 56 are input to current/voltage converters 59 and 60 for converting into voltage signals which are supplied to a differential amplifier 61 and output as a photomagnetic information signal.
The reason for generally using a differential detection optical system comprising a differential amplifier for extracting photomagnetic information signals is that noise is decreased. The photomagnetic information signals have the following types of noise: (1) Noise generated by variations in the intensity of a light source and variations in the film surface reflectance of a medium surface; (2) noise generated by nonuniformity in the medium or on the medium surface, e.g., grain boundaries, defects, dust, etc., (3) noise generated by the unpolarized component of light which is produced due to incomplete polarization of a polarizer/analyzer; (4) noise generated by incomplete focusing or tracking; (5) shot noise of the photodetector; (6) amplifier noise; and so on.
These noises can significantly be decreased by using the differential detection optical system because the in-phase components of electric signals output from the photodetectors are cancelled. In this way, the differential detection optical system for cancelling the in-phase components of the electric signals is important for detecting the photomagnetic information signals.
However, since a conventional differential detection optical system uses a differential amplifier, the system has a problem in that the detection characteristics thereof greatly depend upon the characteristics of the differential amplifier, i.e., noise characteristics over a wide range from a low-frequency region to a high-frequency region.
For example, the photomagnetic signal detection circuit disclosed in Japanese Patent Laid-Open No. 2-206048 thus comprises a differential detection optical system which uses no differential amplifier for detecting photomagnetic information signals. The photomagnetic signal detection circuit disclosed in Japanese Patent Laid-Open No. 2-206048 uses two photodetectors which are connected in series so as to detect photomagnetic information signals by obtaining the operating output from the connection point therebetween.
However, in this differential detection optical system comprising the two photodetectors which are connected in series, since the currents of the low-frequency components of the currents generated in the two photodetectors flow into a GND or a power source through the different photodetectors, variations occur in the DC level at the connection point between the two photodetectors due to variations and unbalance in the input light, thereby causing the possibility that level variations occur in the level of the photomagnetic information signal.